Most prior art image sensing devices operate by projecting an image that is to be scanned onto an array of discrete image sensor elements (usually p-i-n diodes). The projected image is then measured by interrogating the state of each of the sensor elements. For example, FIG. 1 shows a 4.times.5 sensor element section of an array 10 onto which an image having an edge 12 is projected. The term edge is used herein to mean the border defined by light illuminated areas and areas under darker conditions. It is assumed that the area of the array 10 above the edge 12 is illuminated, while the area below the edge is dark.
The twenty sensor elements, shown as the twenty squares 14, are organized into rows A through D, and columns R through V. To scan the image, the illumination state of each of the sensor elements is usually determined using matrix addressing techniques. If a particular sensor element is sufficiently illuminated, for example the sensor element 14 at row A, column R, the charge accumulating on the sensor element since the last read is sensed as being at a first state (ON). If a particular sensor element has not been sufficiently illuminated since the last read, say the sensor element 16 at row D, column V, that sensor element is sensed as being in a second state (OFF). Such sensor elements are called binary sensor elements. If a particular sensor element is partially illuminated, its state depends upon such things as how much of the sensor element was illuminated, the intensity of that illumination, and the duration of that illumination.
An interrogation of all of the illustrated sensor elements of the array 10 results in the rather coarse approximation to the image as shown in FIG. 1, with the ON state sensor elements in white and the OFF state sensor elements in crosshatch. This representation results from a binary thresholding of the pixel (sensor element) values. An alternative prior art array uses sensor element which are capable of reading values of the average illumination applied to that sensor element, the so called gray scale sensor elements. With either gray scale or binary sensor element, the edge position information within a pixel is converted into a spatial average.
It is highly desirable to have sensor arrays which match the human visual system's ability to sense edges. That ability is called hyperacuity since the human visual system senses edge positions much better than it does most other image characteristics. With the prior art sensor elements described above, any increase in the accuracy of the imaging of edge position, or equivalently, any improvement in hyperacuity, smaller and more numerous sensor elements are required. However, fabricating closely spaced, but isolated, sensor elements becomes excessively difficult as the sensor element density increases sufficiently to approximate to the hyperacuity capability of the human visual system.
In addition to the discrete sensor elements described above, another type of sensor element, called a position sensitive detector, is known in the prior art. An example of a position sensitive detector is the detector 20 shown in FIG. 2. The detector 20 outputs photoinduced analog currents into lines 22, 24, 26, and 28. Those currents can be used to determine the position of the centroid of the illuminating light spot 30. The centroid of the light spot in the x-direction (horizontal) can be computed from the quantity (I.sub.26 -I.sub.28)/(I.sub.26 +I.sub.28), while the centroid of the light spot in the y-direction (vertical) can be computed from (I.sub.22 -I.sub.24)/(I.sub.22 +I.sub.24), where I.sub.2x is the current on one of the lines. At least partially because position sensitive detectors are usually rather large (say from about 1 cm.times.1 cm to 5 cm.times.5 cm), they have not been used in imaging arrays.
Even with sensor element density levels far below those required to match the hyperacuity ability of the human visual system, the multiplexing of the sensor element signals from an array of sensor elements onto external access lines is very important. When multiplexing, the ability to integrate the response of each sensor element during time periods between reads is important since integration maximizes the sensor element's signal-to-noise ratio. While multiplexing large numbers of sensor elements of any type is difficult, position sensors are particularly difficult to multiplex. This is because the outputs of position sensitive detectors are preferably currents. Therefore, the lines 22 through 28 are held near ground during reading. Integration therefore requires a large number of dedicated readout channels which is contrary to the use of multiplexing. A large number of readout channels adds considerable cost and complexity to the array.
As discussed above, the acuity of prior art imaging scanners is limited by the density of the individual sensor elements which comprise the scanner's sensor array. This limitation has been overcome by a new type of small sensor elements which is capable of ascertaining spatial information about the light falling within individual elements. Such sensor elements, which are somewhat related to position sensors, are described in co-pending U.S. patent application Ser. No. 08/152,044 entitled, "Hyperacuity Sensing." That patent application is hereby incorporated by reference. Unfortunately, those sensor elements are not easily multiplexed.
Because of the difficulties of achieving high spatial resolution of edges by increasing the pixel density, and the difficulty of using position sensitive type detectors in multiplexed applications, a new type of imaging sensor element which is capable of subpixel edge position accuracy and which can easily be multiplexed would be beneficial.